This invention relates generally to earth boring percussion bits. More particularly, the invention relates to retention systems for ensuring that even when broken during use, the percussion bit remains connected to the drill string.
U.S. Pat. No. 5,065,827 discloses a conventional hammer and percussion bit assembly having a percussion bit retention system. The percussion bit retention system includes a retainer which is mounted on the drive sub of the percussion bit assembly. The retainer includes a lower section having an inner chamber and a thread-shaped, inwardly extending projection. The bottom portion of the percussion bit includes a head section, a threaded section and a recess disposed intermediate the head section and the threaded section. The percussion bit is installed by inserting the upper portion of the percussion bit into the retainer until the threaded section of the lower portion abuts the thread-shaped projection of the retainer. The projection is brought into threaded engagement with the threaded section of the retainer and the retainer is rotated until the projection passes completely through the threaded section of the retainer and enters the chamber. If the percussion bit breaks in the shaft portion above the bottom portion of the percussion bit, the projection will be captured within the chamber by the threaded section of the retainer.
The threaded section of such retainers may become fouled during drilling operations, making removal of the percussion bit difficult. Bits may be difficult to install due to corrosion and wear induced degradation of the threaded section of the retainer.
With the present invention, a novel percussion bit assembly, and associated novel bit retainer member and method of assembling the bit retainer to a percussion bit drill string, can be simply achieved without the requirement for rotating the bit relative to the retainer, and without the risk of thread fouling.
In accordance with the invention, a substantially tubular retainer member is provided, having inwardly extending upper and lower lips and a middle portion longitudinally extending between the upper and lower lips. Multiple slots extend from the lower end of the retainer member to a terminal end disposed intermediate the upper and lower ends. The slots form multiple longitudinally extending cantilever beam segments. The percussion bit is connected to the chuck member by inserting the shoulder of the percussion bit into the middle portion of the retainer member. The cantilever beam segments are resiliently, radially deflected by the shoulder of the percussion bit during installation, the outside diameter of the shoulder being greater than the inside diameter of the lower lip.
The upper portion of the chuck member is inserted through the retainer member and the cantilever beam segments are resiliently, radially deflected to insert of the lower portion of the chuck member into the middle portion of the retainer member. The chuck member is mounted to the housing such that the upper lip of the retainer member is clamped between the lower end portion of the housing and the lower portion of the chuck member. An inner ramp face on the lower lip of the retainer member slides up an outer ramp face in the lower portion of the chuck member and an outer ramp face on the shoulder of the bit to facilitate deflection of the cantilever beam segments.
The upper bearing surface of the lower lip forms an acute angle xcex1 with the interior surface and the lower surface of the shoulder forms an acute angle xcex8 with the surface of the bit, angle xcex8 being complementary to angle xcex1. Preferably, angles xcex1 and xcex8 are substantially equal to 60 degrees.
In a preferred embodiment, the terminal end of each slot defines a substantially circular opening. The length of each slot is preferably greater than half the length of the retainer member.
It is thus an object of the invention to provide a new and improved retention system and assembly method for a percussion bit assembly.
It is also an object of the invention to provide a retention system and method for a percussion bit assembly that does not require threaded engagement between the percussion bit and the other components of the retention system.
Other objects and advantages of the invention will become apparent from the drawings and specification.